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MP3 encoder for browsers using LAME
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What is it?

Mp3LameEncoder.js is a JavaScript library that encodes audio data to MP3 (MPEG-1 audio layer III) on web browsers.

LAME is used for encoding engine. Emscripten is used to convert LAME C code into JavaScript.


It was originally a fork of libmp3lame.js ( API has been totally re-designed and simplified. Now it has been a lower layer part of WebAudioRecorder.js.


Library files

lib/ contains library files.

  • Mp3LameEncoder.js: JavaScript library (uncompressed)
  • Mp3LameEncoder.min.js: JavaScript library (minified)
  • Mp3LameEncoder.min.js.mem: memory initializer data for Mp3LameEncoder.min.js

Using library

Uncompressesd library is a single file. You can use it from both HTML and Web Worker.

  • from HTML: <script src="javascripts/Mp3LameEncoder.js"></script>
  • from Worker: importScripts("javascripts/Mp3LameEncoder.js");

Using minified library is same way. But you must pay attention to memory initializer location.

  • from HTML: default directory is same as HTML which loads Mp3LameEncoder.min.js
  • from Worker: default directory is same as Mp3LameEncoder.min.js path

To change memory initializer path from HTML:

// default path is on the same directory as this HTML
Mp3LameEncoderConfig = {
  memoryInitializerPrefixURL: "javascripts/"   // must end with slash
  // => changed to javascripts/Mp3LameEncoder.min.js.mem
<script src="javascripts/Mp3LameEncoder.min.js"></script>

From Worker:

// default path is on the same directory as Mp3LameEncoder.min.js
self.Mp3LameEncoderConfig = {
  memoryInitializerPrefixURL: "javascripts/memory/"
  // => changed to javascripts/memory/Mp3LameEncoder.min.js.mem


encoder = new Mp3LameEncoder(sampleRate, bitRate)

Create an encoder object.

  • Parameters
    • samleRate: sampling rate [Hz]
    • bitRate: MP3 bit rate [kbps]
  • Returns
    • encoder object

Actual bit rate is set as follows (at 44100Hz).

   .. 72 ->  64
 73.. 89 ->  80
 90..104 ->  96
105..120 -> 112
121..144 -> 128
145..176 -> 160
177..208 -> 192
209..240 -> 224
241..288 -> 256
289..    -> 320

Current implementation supports LAME CBR encoding only.


Encode audio buffers.

  • Parameters
    • buffers: array of sample buffers ([Float32Array, Float32Array])
  • Returns
    • (none)

buffers must be an array of two Float32Array audio data (separate strereo, range = [-1, 1]). It supports stream (incremental) processing. Sample buffers are processed to MP3 stream and appended to internal data.

blob = encoder.finish([mimeType])

Finish encoding and get MP3 as a Blob.

  • Parameters
    • mimeType(optional): MIME type (default = "audio/mpeg")
  • Returns
    • Blob object

After calling .finish(), all internal data is cleared (to avoid resource leak). You must create a new encoder object to encode another audio data.


Cancel encoding and clear all internal data.

  • Parameters
    • (none)
  • Returns
    • (none)

You should call .cancel() manually to avoid resource leak when recording is canceled (encoder's internal memory is not deallocated by unbinding an object variable).


Emscripten and ruby are required to build the library.

$ rake

Download and extract LAME, build library files (see Rakefile for more details).


LAME is licensed under the LGPL. JavaScript-converted part of this library follows the same license as LAME (see below).

JavaScript wrapper API part of this library is released under MIT license (see LICENSE.txt).

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